Galvanic current measuring method and apparatus for monitoring build-up of biological deposits on surfaces of dissimilar metal electrodes immersed in water

ABSTRACT

The build-up of the deposits on surfaces immersed in water in monitored by passing the water through a galvanic cell comprising a pair of dissimilar metal electrodes (5, 6) mounted on opposite sides of a passage (4) for the water in a housing (1,7), and detecting and interpreting galvanic current developed between the electrodes (5, 6).

This invention relates to a monitoring method and apparatus, and inparticular to a method of monitoring the build-up of deposits onsurfaces immersed in water and to apparatus for carrying out suchmethod.

Continuous chlorination of seawater cooling supplies for power stationsduring the summer months is advised to prevent the settlement and growthof species such as mussels which can induce corrosion of copper basedcondenser tubes and restriction of cooling water flow. Since titaniumcame into general use, the incentive to provide continuous chlorinationfor this purpose has diminished. However, chlorination or some otherform of biocidal treatment is still required to control the growth ofbacterial slime films which form the basis of a micro-fouling layercontaining larger organisms, silt, etc. Preventing the formation of theslime film improves heat transfer and reduces the resistance to flow.

There is growing legislative pressure in the United States of Americaand the EEC to reduce or eliminate the discharge of active chlorine frompower stations. This fact, together with a desire for economy in the useof chlorine (or sodium hypochlorite) and the lack of detailed knowledgeon the change in the rate of biofouling throughout the year, indicatethe need for techniques to monitor the build-up of biofouling films andthe effect of anti-fouling procedures on them.

At present the existence of such films can be established only byinspection at outages or inferred indirectly from their effect oncondenser performance (vacuum) changes, which may be due to other causessuch as steamside fouling of the condenser tubes or maldistribution ofsteam flows and off-gases.

According to this invention there is provided a method of monitoring thebuild-up of deposits on surfaces immersed in water, characterised by thesteps of passing water through a galvanic cell comprising a pair ofdissimilar metal electrodes having means responsive to galvanic currentdeveloped between the electrodes connected across the elect-odes, andinterpreting changes in the galvanic current developed to obtain anindication of the build-up of deposits on the electrodes.

The method of the invention derives from the fact that in water thedegree of galvanic corrosion between metal couples incorporating-noblematerials such as titanium and stainless steel depends- very largely onthe reduction of oxygen on the cathodic surface. This process iscontrolled by the biochemical activity of bacterial species present onthe cathode surface, and until a basic slime film forms very littlecurrent flows.

A rapid increase in galvanic current indicates full development of aslime film which is the precursor of subsequent growth of thickerfouling films.

This invention will now be described by way of example with reference tothe drawing, in which:

FIG. 1 is a sectional view of apparatus for use in carrying out themethod of the invention and is taken along ling I--I in FIG. 2;

FIG. 2 is a longitudinal sectional view through the apparatus of FIG. 1and is taken along line II--II in FIG. 1; and

FIG. 3 is a graph illustrating operation of the apparatus of FIGS. 1 and2.

Referring to the drawings, the apparatus comprises a housing 1 formed bya PVC material tubular member having externally threaded end portions 2by which the housing 1 can be connected to the cooling water system of apower station, through which seawater 100 flows. Located in the housing1 is an insert member 3 of blocked PVC material, which defines a passage4 for the seawater 100 flowing through the apparatus as indicated byarrow F in FIG. 2.

Mounted along opposite sides of the passage 4 and extending the lengththereof are two dissimilar metal plate electrodes 5 and 6 havingsubstantially equal area facing surfaces. The electrode 5 is of navalbrass while the electrode 6 is of titanium. Otherwise electrodes ofsteel and stainless steel can be used.

The insert 3 and electrodes 5 and 6 are retained in the housing 1 by endplates 7 of PVC material.

Each of the electrodes 5 and 6 has connected thereto a contact member 8or 9 which extends out of the housing 1, and a means 10 responsive togalvanic current developed between the electrodes 5 and 6 in use of theapparatus, for example a zero-resistance ammeter, is connected acrossthe contacts 8 and 9.

In use, the apparatus is connected to the cooling seawater system of apower station, and the seawater flows through the passage 4 in thehousing 1 between the electrodes 5 and 6. The apparatus functions as agalvanic cell, with the ammeter 10 indicating the galvanic currentdeveloped between the electrodes 5 and 6. As mentioned above, the valueof the galvanic current developed is dependent upon the build-up ofdeposits, that is the slime film, on the surfaces of the electrodes 5and 6, and changes in the current will be indicated by the ammeter 10.Thus, such changes in the galvanic current can be interpreted to obtainan indication of the extent of deposit build-up whereby it can bedecided when action is necessary to remove such deposits from thecooling water tube in which the apparatus is mounted.

FIG. 3 is a graph illustrating operation of an apparatus as describedabove, with galvanic current developed (μA) being plotted against time(in days) As shown, initially there is little galvanic currentdeveloped, but after two days the current begins to rise to reach amaximum in about ten days, and then falls off to a still relatively highlevel over the next nine days. On day nineteen action is taken to removethe deposits (the addition of chlorine to the seawater), and as clearlyshown this has a rapid effect to reduce the galvanic current to arelatively low level, indicating that the deposits have beensubstantially removed from the electrodes 5 and 6.

The method and apparatus as described above have the advantages thatthey can provide a continuous record of fouling conditions in condensertubes, heat exchangers and cooling water tubes of power stations, andenable controlled action to be taken to remove deposits whereby the useand cost of chemicals and power used for deposit removal can be kept toa minimum with a consequential low level of contamination of theseawater used for cooling.

I claim:
 1. A method of monitoring the build-up of biological depositson surfaces immersed in water, comprising the steps of:continuouslypassing the water through a galvanic cell having a pair of dissimilarmetal electrodes on at least one of which build-up of biologicaldeposits is expected to occur; monitoring galvanic current developedbetween the electrodes; and monitoring changes in the galvanic currentto obtain an indication of the build-up of deposits on the electrodes.2. Apparatus for monitoring the build-up of biological deposits onsurfaces immersed in water, comprising:a housing; a pair of opposedplate members of dissimilar metals within said housing, defining apassage for the water to flow therebetween as the water flow throughsaid housing forming electrodes; and means for monitoring galvaniccurrent developed between the plate electrodes to indicate the build-upof biological deposits on said plate electrodes.
 3. Apparatus as claimedin claim 2, wherein the housing comprises a tubular member open at bothends and adapted for connection in a tube through which water flows. 4.Apparatus as claimed in claim 3, wherein the electrodes extend thelength of the tubular member.
 5. Apparatus as claimed in claim 2,wherein the electrodes have equal area facing surfaces.
 6. Apparatus asclaimed in claim 2, further comprising an insert member within saidhousing and mounting said electrodes and cooperating with saidelectrodes to define the passage.
 7. Apparatus for monitoring thebuild-up of biological deposits on surfaces immersed in water,comprising:a housing; a pair of opposed electrodes of dissimilar metalswithin said housing, having equal-sized facing surfaces, and defining apassage for the water to flow therebetween as the water flows throughsaid housing; and means for monitoring galvanic current developedbetween the electrodes to indicate the build-up of biological depositson said electrodes.
 8. Apparatus as claimed in claim 7, wherein thehousing comprises a tubular member open at both ends and adapted forconnection in a tube through which water flows.
 9. Apparatus as claimedin claim 8, wherein the electrodes extend the length of the tubularmember.
 10. Apparatus as claimed in claim 8, wherein the electrodes areformed as plate electrodes.
 11. Apparatus as claimed in claim 7, furthercomprising an insert member within said housing and mounting saidelectrodes and cooperating with said electrodes to define the passage.